1. Field of the Invention
Embodiments of the invention generally relate to a method for measuring the concentration of components in a plating solution useful in electrochemical plating systems.
2. Description of the Related Art
Metallization of sub-quarter micron sized features is a foundational technology for present and future generations of integrated circuit manufacturing processes. More particularly, in devices such as ultra large scale integration-type devices, i.e., devices having integrated circuits with more than a million logic gates, the multilevel interconnects that lie at the heart of these devices are generally formed by filling high aspect ratio, i.e., greater than about 4:1, interconnect features with a conductive material, such as copper. Conventionally, deposition techniques such as chemical vapor deposition (CVD) and physical vapor deposition (PVD) have been used to fill these interconnect features. However, as the interconnect sizes decrease and aspect ratios increase, void-free interconnect feature fill via conventional metallization techniques becomes increasingly difficult. Therefore, plating techniques, i.e., electrochemical plating (ECP) and electroless plating, have emerged as promising processes for void free filling of sub-quarter micron sized high aspect ratio interconnect features in integrated circuit manufacturing processes.
In an ECP process, for example, sub-quarter micron sized high aspect ratio features formed into the surface of a substrate (or a layer deposited thereon) may be efficiently filled with a conductive material, such as copper. ECP plating processes are generally two stage processes, wherein a seed layer is first formed over the surface features of the substrate (generally through PVD, CVD, atomic layer deposition (ALD), or other deposition process in a separate tool), and then the surface features of the substrate are exposed to an electrolyte solution (in the ECP tool), while an electrical bias is applied between the seed layer and a copper anode positioned within the electrolyte solution. The electrolyte solution is generally rich in copper ions (Cu2+) that are to be plated onto the surface of the substrate, and therefore, the application of the electrical bias, i.e., configuring the substrate as the cathode, causes these ions to be plated onto the seed layer, thus depositing a layer of the ions on the substrate surface that may fill the features.
Generally, ECP electrolytes have both inorganic and organic compounds/components at low concentrations. Typical inorganics include copper sulfate (CuSO4), sulfuric acid (H2SO4), and trace amounts of chloride (Cl−) ions. Other components include accelerators, suppressors, and levelers. An accelerator is sometimes called a brightener or anti-suppressor. A suppressor may be a surfactant or wetting agent, and is sometimes called a carrier. A leveler is also called a grain refiner or an over-plate inhibitor. The sulfuric acid generally operates to adjust the acidity/pH and conductivity of the solution, while the copper chloride provides negative chlorine ions needed for proper action of suppressor molecules and facilitates proper anode dissolution.
Although simple in principle, copper plating relies in practice on the use of proper components in the electrolyte to determine the properties of the copper being deposited. Because of depletion, analysis of the processing components is required periodically during the plating process. If the concentrations change, or if the components get out of balance, the quality of the plated copper deteriorates. In addition, the depletion of certain components is not generally constant over time, nor is it generally possible to correlate the plating solution composition with the duration of the plating solution use. Thus, the component concentrations may eventually exceed or fall below a tolerance range for optimal and controllable plating. It is very important for ECP systems to monitor and control concentrations of inorganic and organic components, especially as the technological demands on the copper become more stringent.
Chemical analyzers implementing different analytical principles such as end-point titration and back titration, and others, are used to analyze the concentrations of components, such as dissolved copper ions, in metal plating baths. The chemical analyzer is typically coupled to a metal plating apparatus, such as an electrochemical plating (ECP) apparatus for depositing metal films on semiconductor devices. Similarly, these analytical principles can be applied to manually analyze component concentrations.
“Titration” for measurement of copper concentration is accomplished through adding a quantity of a known concentration of reactants that reacts with the copper. The progress of the reaction is measured by the amount of reaction product produced by the chemical reaction between copper and the reactants, and an end point can be detected and correlated to a copper concentration in the electrolyte. The titration method generally requires two or more reagent solutions in two or more steps, for example, at least one chelating agent solution to titrate the metal ions, such as copper, aluminum, and others, and at least another pH-buffering agent solution, such as an ammonium hydroxide solution, to keep the pH in an effective range for metal chelating reaction to occur. If this is not done, then most chelating agents combine or react with the metal ions, such as copper, impractically slowly such that the complete reaction time for each reaction adds up to a impractically long analysis time.
Another method that may be used is a “back titration” method, which employs an excess amount of a chelating agent solution for a first chelating reaction to occur in one waiting period rather than multiple waiting periods for the reaction to complete, and another titrating reagent solution to react with the excess amounts of the chelating agent in the first chelating agent solution or with the amount of by-products (e.g. acids, bases, aggregates, precipitates) formed after the first chelating reaction. Again, two or more reagent solutions are needed. Such titrating reagent may react faster with the chelating agent than the metal ions to be measured. Suitable titrating reagent solution includes a solution having metal ions, such as zinc ions, to titrate the excess chelating agent, and other buffers, such as a hydroxide solution (e.g. sodium hydroxide, etc.), to titrate the pH back to the original pH. Another example is to use an excess amount of a potassium iodide solution to reduce or convert copper ions from Cu+2 to Cu+1, thus creating an amount of iodine equal to the initial copper II ions (Cu+2) present. Since this reaction is relatively slow and not reliably measured, a titrating reagent solution such as a sodium thiosulfate solution is then used to oxidize the iodine back to iodide ion, a reaction that can be repeatably detected. For back titration, the concentration of the unknown electrolyte component can be measured by considering the excess amount of the chelating agent in the first chelating solution and the required amount of titrating reagent in the titrating reagent solution. One problem is that the reaction by-products, such as precipitates, released acids, or others, may cause build up in the chemical analyzer or interfere with any of the on-going chemical reactions. In addition, titration and back titration methods have the limitations in that they use more chemicals, are time consuming for different reactions to occur, are subject to fluctuation of the pH of different solutions that may not be optimal for the different reactions to occur, and have a lack of a sharp or definable reaction end point.
Therefore, a need exists to provide methods and reagents for real-time analysis of electrolyte components in a processing system, either manually or through the integration of one or more chemical analyzers.